The present invention relates to the general art of electrical systems and devices, and to the particular field of discharging or preventing accumulation of electric charges.
The problem of electrostatic discharge (ESD) is well known. From merely receiving a mild shock after crossing a room and touching a metal object, to sending a shock into electronic equipment, nearly everyone has experienced an ESD problem at some time.
While static electricity is extremely complex, several overall theories are generally accepted with regard to the action of ESD. Static electricity charges on a person or object are generally like charges. As such, as static electricity charges build up on a person or object, these charges tend to migrate as far apart from each other as possible as determined by the geometry of the person of object. Thus, for example, it is common for static electricity charges to migrate to a person""s fingertips. For this reason, when that person reaches out to touch an electrically conductive object, a spark will jump when the gap between that person""s fingertips and the object based upon the potential difference between the fingertips and the object. This discharge is very rapid and can be quite violent. If the electrically conductive object is sensitive electronic equipment, that equipment may be damaged either from the magnitude of the discharge and/or from the speed of the discharge. At the least, the charge could cause the equipment to execute an error. A sufficient number of such discharges may eventually damage the equipment.
Accordingly, the art contains many inventions intended to protect the equipment or the person from the effects of this sudden, and sometimes violent, discharge associated with ESD.
For instance, in the logging industry where chains are lowered by helicopter to loggers waiting on the ground to fasten fresh-cut timber to them so it could be airlifted to the sawmill or nearby waterway, track access point or the like, the loggers are often reluctant to grab the chain because of a painful shock that may occur as a result of a buildup of static electricity which will be discharged to ground through their bodies. This particular problem has been solved by incorporating a resistance in the line from the charge-carrying object, such as the helicopter, to the person on the ground. The high resistance causes the current to be low enough that the discharge will not be painful.
However, this is cumbersome. This solution may be even more cumbersome if the person is an office worker who moves around a great deal. Accordingly, this solution to the ESD problem has serious shortcomings.
Accordingly, there is a need for a system that protects a person against the effects of ESD but can do so in a manner that does not interfere with any task the person may be performing and further will not be cumbersome or burdensome for the person to use.
Still other inventions are directed to protecting electronic equipment from the effects of ESD. For example, many computers include touch pads or touch areas for the user to touch before touching the remainder of the computer. The touch pads are grounded so the ESD will pass from the person via a spark or the like directly to ground without going to or through the computer.
While many of these devices work well, there are several problem areas not addressed thereby. This results in drawbacks and disadvantages for such devices when a person or equipment are situated in certain environments or subject to certain conditions.
First, no matter how effective a touch pad is it will be totally ineffective if the person does not use it. That is, if the person carrying a large ESD charge forgets to touch the touch pad and proceeds to touch a computer, the ESD will discharge through the computer and the touch pad will have been useless. Thus, a shortcoming of such touch pads is that they require the person to remember to use it.
Furthermore, no matter how effective the ESD protection device is, the current level and/or the change in current level may be so high that either the person or the equipment can be damaged.
Still further, while placing a touch pad on a computer may protect the computer it does not protect the user from the effects of an electrostatic discharge.
As mentioned above, the majority of applications for the prevention of ESD are in the manufacturing or medical fields and are largely concerned with protecting the xe2x80x98manufacturingxe2x80x99 process or sensitive components for ESD damage. Examples include moving mediums such as the manufacture of rolls of paper, the assembly of delicate electronic chips and circuitry and surgeon-patient contact during an operation.
An analysis of each of the above will help illustrate the shortcomings of the prior art. In the manufacture or printing of paper, long rolls of paper may move at high speed. Often the path may involve rubber or other rollers and guides. As the paper rubs across such items a static electricity charge may be generated. Since the paper path is well controlled, it is an easy process to place grounded conductive brushes or flat metal springs in contact with the moving paper since the paper stays in a fixed path. Such electrodes are connected directly to the grounded frame of the associated machinery or to another path eventually leading to earth ground or other equalizing means.
Another common application of ESD control is in the production or repair of fragile electronics such as computer circuit boards. Even a slight electrostatic discharge through a sensitive device may destroy it. Therefore, significant effort and cost is devoted to eliminating the possibilities of electrostatic potentials in the vicinity of the sensitive electronics. Typically, a single ground point is provided that all associated elements are connected to so that no electrostatic potential can exist between them that might flow through the sensitive electronics. For example, an assembly person is connected to a ground, typically by a wrist strap tether. The tether generally consists of a wrist pad and grounding wire that is eventually connected back to the single ground point. For operator safety, the ground wire typically contains a 1 Meg resistor to limit current flow to safe levels should the operator come in contact with 120 volts AC. This tethering restraint is inconvenient and not considered suitable for a typical office worker or call center operator. The single ground point is eventually connected to true earth ground or other equalizing point by another conductor.
Applications are similar in the medical field, employing similar tethers and/or foot/shoe connectors also considered impractical for the typical office worker environment.
Today, a new set of ESD problems is emerging in the typical work place or home office environment. Today, a typical worker may exist in a virtually electrically isolated environmentxe2x80x94a plastic computer case, plastic keyboard, plastic control knobs on a molded plastic control panel, plastic office chair with man-made fabric and plastic wheels, non-conductive flooring or carpeting and even a headset with foam or molded plastic earpieces and plastic microphone tube.
As the operator moves in his/her chair, there are many opportunities for a very large electrostatic charge to build up on his/her body. Friction between dissimilar materials is the classical means for generation of electrostatic voltages. There are many such situations that exist continually in the operator environment todayxe2x80x94the operator""s clothing sliding against the chair back or arm rests, the operator""s shoes sliding on the carpet, the plastic chair wheels sliding against the carpet are a few examples. The effects can be cumulative over a long period of time, and can become quite high.
Eventually a discharge or equalization to (true earth) ground must take place. The higher the value of the electrostatic voltage charge, the greater the distance the charge may xe2x80x98jumpxe2x80x99 to discharge, and the more xe2x80x98catastrophicxe2x80x99 the event to the operator. For example, there are many documented cases of operators in call centers experiencing a very loud pop or explosion in their ear, ear pain, and even bleeding in the ear as the discharge path appears to take place through the operator""s headset. Other documented cases include severe neck pain, nausea, numbness, elevated blood pressure and rapid heart beat.
In an attempt to mitigate these effects, xe2x80x9canti staticxe2x80x9d carpet may often be installed in the initial design or construction of a workplace.
There are a number of different types of carpet intended to reduce the disturbing effects of electrostatic charge build up on a person""s body. They are designed to be xe2x80x98partiallyxe2x80x99 electrically conductive or dissipative to reduce the effects of ESD (Electrostatic Discharge) by draining the charge from a person""s body via shoes or specially worn metal heel plates which may then have a conductive strap to the person""s skin. The term dissipative in ESD terminology typically refers a relatively high resistance to ground, such as 100 megohms. Although this sounds like a high value, it is effective for draining static charge as compared to many floorings, adhesives, and especially underfloorings that are considered as insulators and not effective for ESD.
Proper performance of these carpets depends on proper installation which may typically include grounding by a metal plate perhaps every 10 to 30 feet or other conductive underflooring means.
Often, the anti-static carpet appears not to work, even when samples of it test within spec. Or, it may work in part of a facility but not elsewhere. This may be evidenced by most static shocks taking place in one area of a facility. Investigation in such cases typically reveals a grounding difference between the areas. The troubled area may be an add-on area where the same carpet was ordered but the installer didn""t follow instructions, or the troubled area may have an insulating plywood underfloor or raised floor that is not conductive.
Unfortunately re-installation of the carpeting is not feasible in many office environments. For example, modular office partitions may be sitting on the carpet, along with their load of wall hanging cabinets, electrical and network wiring, file cabinets, chairs, etc. Compounding the problem are the facts that office modules are usually small, further raising the density of furniture and an electrician might be needed to remove module wiring so the units could be disassembled. Thus attempting to reinstall the carpet may be almost as major as a building remodeling.
As users in the workplace become concerned about ESD, they may be sensitive to every logical source and solution. This sensitivity often focuses on certain items such as the plastic face plate covering many CRT monitors. Running a hand over the face plate may draw a static charge that may be viewed by the user as one of the sources of static electricity buildup to be concerned about.
There are many possibilities as to why these effects are worse than the typical nuisance static electricity charged walking around the house. For instance, the discharge path may be more surprising or appear worse to the user if it involves the user""s ear. Recently, this has been attributed to electrostatic discharge of the operator with the grounding mechanism being the metallic portion of the ear piece coupled to its metallic conductors and eventually to earth ground through its associated electronics. This may be a direct low impedance ground or it might be a higher impedance which is still sufficiently low with respect to that needed to successfully equalize the static charge. Still in other cases, as explained below, the associated electronics may potentially make the discharge injury to the person more severe and disturbing by causing a high current pulse to take place as the discharge event.
In some cases, the associated electronics may experience physical damage or processing disturbances due to the operator electrostatic discharge. For example, the headphone circuit might involve a transformer with a 600 to 10,000 volt breakdown rating between its windings (connected to the headset diaphragm) and conductive metal core. However, the electrostatic voltage on the operator may exceed 15,000 volts-far more than the design tolerance of the transformer. Should the transformer be exposed to such excessive high voltage, a xe2x80x98breakdownxe2x80x99 or xe2x80x98shortingxe2x80x99 may occur. Thus, the operator electrostatic voltage might cause a xe2x80x98short circuitxe2x80x99 insulation breakdown or lower resistance to develop between the headset winding (secondary) and primary winding which may be at a constant high voltage level with respect to ground or the transformer core which may be connected to earth ground, thus completing the discharge path.
The transformer breakdown may cause a permanent equipment failure. Other equipment damage or errors can also occur due to the electrostatic discharge event. The electrostatic discharge event may cause an electromagnetic or radio frequency pulse to be generated. This pulse may radiate into nearby circuitry causing errors in processing or noise in audio or video circuits. Although a transformer discharge event has been described above, other similar discharge paths can be envisioned, with similar catastrophic results.
With continued miniaturization of electronics, the problems may become more severe as circuit component voltage tolerances become less and enclosure insulation distances become less.
Accordingly, there is a need for an ESD protection system that protects a person as well as sensitive electronics from the effects of ESD, even if that person is in an environment that is intended to nominally insulate that person or electronic equipment from ESD.
As the cost of doing business increases, many businesses are reluctant to purchase new original equipment. Thus, it is most advantageous if existing equipment can be easily modified or retrofit to achieve new and improved results. This is the situation with protecting people from the effects of ESD. Thus, there is a need for a system for protecting people against the effects of ESD that can easily be retrofit onto existing equipment.
When grounding ESD it is also important that a disrupting or damaging charge not be induced or superimposed back into nearby sensitive electronics. In the case of modular furniture, there may only be one ground wire present which is wired to each one of the electrical outlets located in the modular furniture. This ground wire becomes the source of ground for each and every piece of attached electronics, such as PC""s at each work station all connected together in a network. The network may be extended to PC""s located in various rooms or large groups. Signal voltages are very small on the network, and the network may xe2x80x98followxe2x80x99 the ground paths. There is also resistance or impedance in any ground network. This may be especially true on grounding conductors used in modular furniture or office raceway systems since space is limited and conductors are usually just large enough to meet the minimums of the National Electric Code. Therefore, any significant ground current from ESD may be converted to a relatively high voltage as compared to signal levels. As indicated in FIG. 11A, circuitry sneak paths often exist at each connected device in a network environment between the signal paths and ground. ESD discharge currents in the ground path may generate a potential and this is a voltage that may be superimposed on the PC""s and network causing disruption or damage by interfering with (relatively minute) signal voltages. By a similar process noise may be caused which is widespread into phone systems such as in a call center or cause noise in studio audio and video systems. Other similar processes will be obvious to one skilled in the art such as automation, laboratories, control centers, broadcast studios, etc.
Therefore, there is a need for a means of grounding electrostatic discharge currents for grounding equipment so that sneak paths present do not develop disturbing voltages or currents.
It is a main object of the present invention to protect a person from the effects of ESD.
It is another object of the present invention to protect a person from the effects of ESD without requiring that person to wear any cumbersome wearing apparel.
It is another object of the present invention to protect a person from the effects of ESD without requiring the person to remember to carry out any special operation.
It is another object of the present invention to prevent or reduce an uncomfortable, disturbing or harmful electrostatic discharge to a person.
It is another object of the present invention to prevent or reduce an electrostatic discharge that might interfere with a person""s ability to carry out his or her job.
It is another object of the present invention to provide a system to equalize (drain or discharge) an electrostatic charge from a person in a safe, harmless, nonnoticeable or minimally noticeable manner.
It is another object of the present invention to provide a contact surface to the person that is compatible with their normal (workspace) environment and provides discharge contact in the normal course of the operator""s activities.
It is another object of the present invention to minimize the static shock that may take place upon initial contact by a prior-charged person.
It is another object of the present invention to provide a carefully controlled ground discharge path.
It is another object of the present invention to provide a ground discharge path that minimizes radiated disturbances to nearby equipment.
It is another object of the present invention to provide a discharge path that minimizes conducted disturbances to interconnected equipment.
It is another object of the present invention to provide a convenient earthing or equalization means.
It is another object of the present invention to support other work necessities of the operator such as operation of a computer mouse, keyboard, track ball or similar needs.
It is another object of the present invention to provide a grounding means via an existing ground of the associated electronics limiting the discharge current to a minimal value and waveshape so the operation of the associated electronics is not degraded.
It is another object of the present invention to provide an ESD suitable grounding means via existing signal conductors of the associated electronics limiting the discharge current to a minimal value and waveshape so the operation of the associated electronics is not degraded.
It is another object of the present invention to provide a convenient connection means for retrofit or connection in the field to existing systems by an unskilled person.
It is another object of the present invention to provide a convenient means for grounding equipment so that disruptive sneak path currents are not created.
It is another object of the present invention to provide a system for grounding electrostatic discharge current for grounding equipment so that any sneak paths present do not develop disturbing voltages or currents.
It is another object of the present invention to prevent equipment damage or operational disruption due to ESD from a person.
It is another object of the present invention to provide a system that will protect a person from the effects of ESD and which can be used in connection with an electrical plug.
It is another object of the invention to provide a means of grounding anti-static carpet or other applicable flooring materials without requiring upheaval of the workplace.
It is another object of the invention to provide a means of grounding anti-static carpet or other applicable flooring materials without requiring deinstallation and reinstallation of the carpeting.
It is another object of the invention to provide a means of grounding anti-static carpet or other applicable flooring materials by using file cabinets, desks, modular office partitions, other equipment, or the carpet itself to aid in providing sustained mechanical contact for grounding purposes.
It is another object of the invention to provide a means of grounding anti-static carpet or other applicable flooring materials in a safe manner should AC volt such as 120 volts be present.
It is another object of the invention to provide a means of grounding anti-static carpet or other applicable flooring materials to selected ground sources to achieve single point (equipotential) grounding with other office equipment.
It is another object of the invention to provide a means of grounding anti-static carpet or other applicable flooring materials with large surface area contacts commensurate with surrounding objects.
It is another object of the invention to provide a means of grounding anti-static carpet or other applicable materials insulated from random grounds or other objects.
It is another object of the invention to provide a means of grounding anti-static carpet or other applicable materials such that nearby sensitive electronic equipment is not disturbed by conducted or radiated emission as ESD takes place.
It is another object of the invention to provide a means of reducing the electrostatic charge perceived to build up on CRT display monitors.
It is another object of the invention to provide an ESD protection system that protects sensitive electronics from the effects of ESD, even if the equipment is in an environment that is intended to nominally insulate that electronic equipment from ESD.
It is another object of the present invention to provide an ESD protection system that can be easily used with existing equipment.
It is another object of the present invention to provide an ESD protection system that can be used in a layered format.
These and other objects are achieved by a system which augments the grounding of a flooring system such as anti-static carpet without the trauma of reinstallation of the carpet. Reinstallation can be a major problem, especially if the carpeting is covered with tens or even hundreds of modular workstation cubicles complete with file cabinets, PC""s, etc.
Grounding is provided by conductive surfaces in intimate contact with the carpeting. The conductive surfaces are configured to be compatible with the surroundings and in such cases to use other objects as weights to enhance the conductor/carpet contact.
The conductive surface may be insulated as needed to avoid contacts with unwanted grounds. A circuit connects the conductive surface to the desired ground. This may be a plug arrangement as defined in a copending application. Circuitry may be included for safety against AC voltage shock and to prevent radiation or current during ESD from disturbing sensitive electronics.
Optional grounding of CRT monitor face plates which often harbor a static charge is provided in a similar manner.
It is noted that for purposes of this disclosure, the word conductive will be used to refer to the electrostatic discharge contact area being discussed in the general sense unless otherwise noted. In the true sense of the ESD definition that term means all but insulators. In the strict ESD discussion, conduction typically refers to resistances of 0 to 0.1 megohm, dissipative typically refers to 0.1 megohm to about 1012 ohms, and above that as insulative or non-conductive. A combination of two or more of the following is provided: convenient personnel contact means, current limiting means, and grounding path are provided. The current limiting means may contain series elements of high resistance and/or inductance. The inductance is to limit the development of radiated or conducted high frequency, high impulse leading edges of current or voltage which may upset or damage nearby or connected electronics. The high series resistance further limits total current to a value such that static electric charges are not significant if superimposed upon logic or logic ground conductors. The high resistance also limits current flow to the user if instead of ground the ground wire comes in contact with a high voltage source. The value of resistance can be quite high since the goal is merely to reduce the static charges to low levels (for example 500 volts or less) on a periodic basis of minutes or hours as the charge is built up. However, the large resistance prevents a person from receiving a shock if they have a charge when they initially contact the contact element. That is, the large resistance is a balance between actually connecting a contact point to a grounding circuit (which would provide a path for a walk-up shock to occur) and preventing a walkup shock by xe2x80x9cisolatingxe2x80x9d the contact device from the grounding circuit. Points of contact to the user are designed to be those that the user touches continually or intermittently in the normal course of operation so that bleeding of ESD can occur on a continual basis. Examples may include conductive mouse pads, conductive elements on the surface of a computer mouse or computer keyboard, trackball, conductive knobs or elements on a mic mixing panel, conductive elements on the headband or earpieces of a headset worn by the user, often used controls or touch points on virtually any type of user-operated equipment. A convenient earthing means is provided to drain off the static current so the charge may be effectively equalized.
If discharge methods are not used, the static discharge might build up for minutes or hours reaching very high values. The eventual uncontrolled discharge might take place in the user""s ear to the metallic diaphragm inside the earpiece. The event may be frightening to the user.